SiC has excellent characteristics with respect to band gap, dielectric breakdown voltage, electron saturation rate, thermal conductivity and the like. Therefore, SiC has come to be expected as a material for next-generation power devices and high-temperature devices which surpasses the limits of Si, and, attendant on this, developments of substrate materials have been conducted vigorously.
As a method for growth of SiC single crystals, there have been known the sublimation method, the CVD method, the Acheson method, the solution method and the like.
The Acheson method is a method which has long been practiced industrially, wherein silicic anhydride and carbon are heated at a high temperature to cause precipitation of a SiC single crystal. By this method, however, it is difficult to produce a single crystal with high purity. The sublimation method is a method wherein a SiC raw material powder is heated to a temperature of 2,200 to 2,400° C. to be once converted into gas of Si, Si2C, SiC2, or the like, and the gas is again precipitated as SiC on a seed crystal at a low temperature. This method is presently the mainstream method for production of SiC bulk single crystals. Since the sublimation method is a vapor phase growth method, however, there is a problem that various defects are liable to be generated in the crystal obtained. Besides, in the CVD method, it is difficult to produce a bulk single crystal, since the raw materials are gaseous components.
The solution method is a method wherein Si or a Si-containing alloy is melted in a graphite crucible, and, further, carbon is eluted also from the graphite crucible, whereby a SiC single crystal is grown through precipitation from the solution containing Si and C onto a seed crystal disposed in a low-temperature zone. In general, the use of a Si melt alone makes it difficult for C to be sufficiently dissolved on a solid solution basis. Therefore, there is adopted a technique for enhancing the solubility of C by using a solution which contains a third element. According to the solution method, it is possible to obtain a single crystal with higher quality having fewer defects, as compared with the case of the sublimation method. On the other hand, however, by the solution method it is difficult to obtain a growth rate as high as that in the sublimation method. In view of this, various investigations have been made on the method of producing a SiC single crystal according to the solution method.
Patent Document 1 (JP-A 2000-264790) discloses a method of growing a SiC single crystal through precipitation by the use of a melt containing Si, C and a transition metal. Besides, a SiC single crystal is grown by the use of a melt of Si—C—M (M is Mn or Ti) in Patent Document 2 (JP-A 2004-002173), a melt of Si—C—M (M is Fe or Co) in Patent Document 3 (JP-A 2006-143555), or a melt of Si—C—Ti—M (M is Co or Mn) in Patent Document 4 (JP-A 2007-076986).
Patent Document 5 (JP-A 2006-321681) discloses a method of growing a SiC single crystal of a desired crystal structure selected from among 15R, 3C and 6H structures, by using a melt obtained by melting raw materials including Si and C and a third element or a compound thereof. As the third element, borides, Sn (15R), Gd (3C), and Al, Dy and La (6H) are mentioned in this document. Patent Document 6 (JP-A 2007-277049) describes the use of a melt obtained by adding a rare earth element and any of Sn, Al and Ge to Si. Here, the addition of the rare earth element is effective in enhancing the solubility of C in the Si melt and thereby enhancing the growth rate of the SiC single crystal. Under the conditions where the growth rate is high, however, turning into a polynuclear state or polycrystalline state is liable to occur at the growth surface. In view of this, the document shows a technique of stably securing even growth by adding Sn, Al or Ge as an element for uniformly activating the growth surface. Patent Document 7 (JP-A 2009-167045) describes the use of a melt of Si—Cr—X (X is Ce or Nd), and shows that simultaneous addition of Cr and X makes it possible to reduce the number of macroscopic defects generated in the SIC single crystal. In addition, Patent Documents 8 and 9 (JP-A 2005-154190 and JP-A 2005-350324) show methods for growing a SiC single crystal, wherein a SiC raw material rod, a solvent, and a seed crystal are stacked sequentially from the lower side, and a temperature gradient is formed at upper and lower end surfaces of the solvent. In this case, use is made of a solvent including Si and an element selected from the group consisting of Y, lanthanoid, the Group I elements, the Group II elements, the Group IIIB elements, and the like.
When a solution containing a rare earth element is used, the solubility of C can be enhanced and the growth rate, which relates to one of the problems encountered in the solution method, is enhanced. When the solubility of C is enhanced, however, there arises a problem that roughening or turning to a polycrystalline state is liable to occur at the growth surface, lowering the quality of the SiC single crystal. For coping with this problem, the addition of an element for suppressing the solubility of C, simultaneously with the addition of the rare earth element, has been tried. However, this approach is disadvantageous in that since the solution becomes a multicomponent system containing further more components, it is difficult to control the composition of the solution, and the manner of crystal growth is liable to be influenced by subtle variations in the crystal growth conditions. Further, as the SiC single crystal grows from the solution, Si and C among the components of the solution are consumed and the composition of the solution is thereby changed, so that the optimum conditions for crystal growth are largely varied with time. Therefore, according to the solution method, it is difficult to produce a SiC single crystal which is long and large in diametral size. Patent Documents 8 and 9 (JP-A 2005-154190 and JP-A 2005-350324) disclose a method in which raw material is supplied from a SiC raw material rod. In this case, the composition of the solvent is not largely fluctuated, but the SiC raw material rod has to be preliminarily prepared, which leads to a raised production cost.
In addition, Patent Document 8 discloses a method of producing a silicon carbide single crystal wherein the composition of solvent includes Si and at least one coexistent element selected from the group consisting of Y, Sc, lanthanoid, the Group I elements, and the Group II elements of the Periodic Table. Patent Document 9 discloses a method of producing a SiC single crystal wherein the composition of solvent includes Si, Y and at least one element selected from the Group IIIB elements of the Periodic Table. In these cases, the solvent composition is not largely fluctuated, but the SiC raw material rod has to be preliminarily prepared, which leads to a higher production cost.